1. Field of Invention
The present invention relates to light emitting devices particularly those having a structure comprising a substrate, a transparent electrode, a layer of light emitting material and a second electrode.
2. Description of Related Art
In such light emitting devices, holes are injected into the light emitting material from one electrode (usually the transparent electrode) and electrons are injected from the other electrode. Electron-hole recombination in the light emitting, or active, material generates light. The generated light is emitted from the device through the transparent electrode.
An object of the present invention is to provide a light emitting device of the above mentioned type wherein the efficiency of the device is improved.
According to a first aspect of the present invention there is provided a light emitting device comprising a substrates a transparent electrode formed on said substrate, a layer of light emitting material provided over the transparent electrode and having at least one corrugated surface, and a further electrode formed over the light emitting material
In a preferred arrangement there is provided a light emitting device comprising a substrate having a corrugated surface, a transparent electrode formed on said corrugated surface, a layer of light emitting material provided over the transparent electrode and a further electrode formed over the light emitting material.
In another preferred arrangement there is provided a light emitting device comprising a substrate, a transparent electrode formed over the substrate, a conductive polymer layer formed over the transparent electrode and having a corrugated surface opposite to a surface facing the transparent electrode, a light emitting material in contact with said corrugated surface and a further electrode formed over the light emitting material.
According to a second aspect of the present invention there is provided a method of manufacturing a light emitting device comprising the steps of providing a substrate, forming a transparent electrode on said substrate, providing a layer of light emitting material over the transparent electrode, arranging for the light emitting surface to have at least one corrugated surface, and forming a further electrode over the light emitting material.
In a preferred method the step of arranging for the light emitting surface to have at least one corrugated surface includes providing a corrugated surface on the substrate.
Another preferred method includes the step of forming a conductive polymer layer over the transparent electrode and in this method the step of arranging for the light emitting surface to have at least one corrugated surface includes providing a corrugated surface on the conductive polymer layer.
In the present invention, very preferably the light emitting material is an organic material.
It has been found that in the conventional devices, the layer of light emitting material acts as a waveguide and a substantial portion of the generated light can be trapped in waveguide modes within the active material. The higher the refractive index of the light emitting material, the larger the proportion of the generated light which is trapped in waveguide modes in the light emitting mate This consideration will thus be of particular significance with the use of organic materials for the light emitting layer, especially if conjugated polymers are used as the active material. This is because organic materials, especially conjugated polymers, have a high refractive index around the wavelength of the light omitted from the organic materials.
It is understood that proposals have been made for the use of corrugated surfaces within certain electronically pumped laser devices. However, such devices have a fundamentally different structure and mode of operation compared with the light emitting devices to which the present invention applies. Moreover, the previous proposals have been of a mainly theoretical nature and have postulated devices which would be extremely difficult, if not impossible, to fabricate in practice.